SP serial console doesn't detach stale faux-mgs clients on an idle console

faux-mgs allows us to attach to a gimlet's serial console via the SP. While faux-mgs is running, it ensures it's regularly sending serial console-related packets (either writes if there's data, or keeepalives if there isn't). On a clean exit, faux-mgs will detach itself from the console. But on a non-clean exit, the SP is responsible for eventually timing out the connection and allowing new attach requests. However, it looks like only check for staleness at https://github.com/oxidecomputer/hubris/blob/1d3f3d594cb55695fdf8c94e086b50498f05f121/task/control-plane-agent/src/mgs_gimlet.rs#L282-L303

This is inside the code path for "the SP has serial console data to send to faux-mgs". If the console is idle (e.g., someone left it sitting at a shell prompt), the SP will never send data, so will never realize the faux-mgs client has gone away. This isn't a major problem, because faux-mgs can forcibly detach any previous session, but it is annoying because it makes it look like there might be someone else on the console when there isn't. A pretty easy fix would be to also check for staleness in serial_console_attach().

Start in task/control-plane-agent/src/mgs_gimlet.rs, reviewing the existing staleness check around lines 282-303 and the serial_console_attach() entry point. Confirm how an idle console differs from the data-send path, then ensure stale faux-mgs clients are recognized while the console is idle. Done means a new attach can proceed after a non-clean client exit without requiring a forced detach.